In manufacturing lenses, plastics are often desirable as construction materials due to their light weight and durability. Plastic lenses also provide relatively economical vision correction. Methods for producing plastic lenses of various prescriptions are well known.
As used herein a "plastic" lens is one fashioned from optical quality resin materials. Prevalent materials for use in fashioning plastic lenses include resin mixtures containing allyl diglycol carbonates, such as "CR-39" sold by PPG Industries. Materials based on allyl diglycol carbonates are advantageous in that they are relatively scratch resistant; they are have relatively low chromatic aberration (i.e., they have Abbe numbers of around 58); and they can be easily edged with conventional edging equipment typically found in small lensmaking operations, for example, to fit a predetermined eyeglass frame (this characteristic will hereinafter be referred to as greater or lesser "ease of edging"). However, lens materials based on allyl diglycol carbonates are disadvantageous in that they must be relatively thick in order to provide necessary impact resistance.
Bisphenol A polycarbonates provide significantly higher impact resistance than allyl diglycol carbonates. These materials, however, are disadvantageous in that: (1) they are scratch prone; (2) they have relatively high chromatic aberration (i.e., they have an Abbe numbers of around 28); and (3) they are prone to flow during edging due to their relatively low degree of crosslinking, causing conventional edging equipment to rapidly "gum up" and preventing a sharp edge from being formed.
In view of the above, it can be appreciated that there is a need in the lensmaking art for a plastic lens material that provides the impact resistance of bisphenol A polycarbonates, while at the same time providing the relatively high scratch resistance, relatively low chromatic aberration, and relatively high ease of edging of allyl diglycol carbonates.